What size aircon do I need?
Air conditioners are sized in BTU (British Thermal Units per hour) or kW of cooling: the bigger and hotter the room, the more you need. Use the calculator, or find your room in the chart below.
How the calculator works
It allows about 480 BTU of cooling per square metre for a room with a standard ceiling, then adds capacity for the things that make a room hotter:
- Afternoon sun. West-facing rooms and rooms with large windows need 10–15% more.
- Room use. Lounges and open-plan areas need about 10% more, offices about 15% (people and computers), kitchens about 30%.
- High ceilings. For ceilings above about 3 m, size up.
- Insulation. Rooms under an uninsulated roof heat up fast — choose the next size up.
We check every size when we quote, so if you are between two sizes, add the one you prefer to your quote list and tell us about the room.
Aircon size chart
| Room size | Typical room | Aircon size | Cooling | Shop |
|---|---|---|---|---|
| 1–18 m² | Bedroom or study | 9 000 BTU | 2.6 kW | See 21 units |
| 19–25 m² | Main bedroom or small lounge | 12 000 BTU | 3.5 kW | See 33 units |
| 26–37 m² | Lounge or small office | 18 000 BTU | 5.3 kW | See 34 units |
| 38–50 m² | Open-plan living area or office | 24 000 BTU | 7.0 kW | See 26 units |
| 51–62 m² | Large open-plan area | 30 000 BTU | 8.8 kW | |
| 63–75 m² | Boardroom or small shop | 36 000 BTU | 10.6 kW | See 13 units |
| 76–100 m² | Shop, restaurant or showroom | 48 000 BTU | 14.1 kW | See 21 units |
| 101–125 m² | Large commercial space | 60 000 BTU | 17.6 kW | See 12 units |
For an average room with some afternoon sun. Larger spaces can use more than one unit, or a ducted system.
BTU and kW: what the numbers mean
One kilowatt of cooling equals about 3 412 BTU per hour, so a 12 000 BTU aircon gives about 3.5 kW of cooling. That is the heat the unit removes from the room, not the electricity it uses: a modern inverter uses roughly a third as much electricity as the cooling it delivers.
Frequently asked questions
How many BTU do I need per square metre?
As a rule of thumb, allow about 480 BTU (about 140 W) of cooling per square metre for a room with a standard ceiling and some afternoon sun. Add 10–15% for a hot, west-facing room or an office, and up to 30% for a kitchen.
What size aircon do I need for a 3 × 4 m bedroom?
A 3 × 4 m bedroom is 12 m², so a 9 000 BTU (2.6 kW) unit is enough in most homes. Choose 12 000 BTU if the room gets strong afternoon sun.
What size aircon do I need for a 5 × 6 m lounge?
A 5 × 6 m lounge is 30 m². An 18 000 BTU (5.3 kW) unit suits most lounges of that size; a shaded room may manage with 12 000 BTU, and an open-plan area with a kitchen may need 24 000 BTU.
Is it bad to buy an aircon that is too big?
An oversized fixed-speed unit cools the room quickly, then switches off before it has removed the humidity, so the room feels clammy and the compressor wears faster. Inverter units cope better because they slow down, but an oversized unit still costs more to buy.
What happens if my aircon is too small?
An undersized unit runs flat out on hot days and may never reach the temperature you set, which uses more electricity and shortens its life. If in doubt between two sizes, choose the larger one for sunny rooms.